1. Field of the Invention
This invention concerns an optical disk apparatus suitable for detecting tracking error signals by the push-pull method.
2. Description of the Prior Art
FIG. 18 shows a conventional optical disk apparatus. A light beam from a laser light source 1 advances via a collimator lens 2, a mirror 3 and an objective lens 4 so as to form a spot on an optical disk OD. Light reflected from the optical disk OD passes through a beam splitter 5 and a condensing lens 6 to a photodetector 7. The objective lens 4 and the mirror 8 are installed in a head 8 which is free to slide in the radial direction of the optical disk, all devices from the laser light source 1 to the beam splitter 5 being fixed.
The photodetector 7, as shown in FIG. 19, comprises two regions A, B divided by a boundary line corresponding to the tangential direction of the optical disk, the difference between the output from these two regions A, B giving a tracking error signal according to the push-pull method.
The distribution of reflected light condensed on the photodetector 7 varies due to diffraction produced by the positional relationship of the pits or grooves and the spot on the optical disk.
The mirror 3 is free to rotate in the direction of the arrow in the FIG. 18, and its angle is controlled by means of the tracking error such that the spot on the optical disk is in the center of a track.
In the aforesaid conventional optical disk apparatus, however, when the mirror 3 is inclined without moving the head 8 so that the spot is moved in a radial direction, the optical path of the reflected light beam is shifted by an amount .delta. compared to the case when the mirror is in its reference position as shown in FIG. 20. The entire spot on the photodetector is then displaced from the reference position shown by the solid line to the position shown by the broken line in FIG. 19, and the tracking error signal will contain an offset.
In this specification, a component of the tracking error signal caused by the shift .delta. of the reflected light is defined as a tracking offset signal.
FIG. 21 is a graph showing the relation between the position of the spot on a disk relative to a track and the tracking error signal. The horizontal axis the shift amount of the center of a spot from the center of a track and the vertical axis the tracking error signal. The -- line in the figure shows the tracking error signal when the shift .delta. of the reflected light is 0.00 mm, the .times.--.times. line shows the tracking error signal when the shift .delta. of the reflected light is 0.19 mm and the .smallcircle.--.smallcircle. line shows the tracking error signal when the shift .delta. of the reflected light is 0.38 mm.
If the optical disk and the light beam are inclined relative to each other, the tracking error signal curve is shifted overall as the mirror inclination angle increases, and the actual shift amount and the signal do not correspond. Even if a servo controller based on the tracking error signal is used, therefore, the position of the spot cannot be accurately controlled.